Agriculture Reference
In-Depth Information
so it is necessary to keep the nozzles clean
(e.g.
8.6.3
Cooling by evapotranspiration
by
submerging
them
in
an
acid
solution).
The simplest method to evaporate water is
to do it through the plants by their transpi-
ration, which involves a non-restrictive
water supply and a good air exchange to
evacuate the exceeding heat. Under
Mediterranean conditions at least 20 vol-
umes h
−1
must be achieved (
R
, the air
exchange rate) for external radiation values
of 700 W m
−2
(Fuchs, 1990).
Low pressure systems (misting)
Their working pressure is lower than
0.5 MPa. The nozzles generate droplets
whose size ranges from 20 to 100 mm, with
flows from 10 to 120 l h
−1
. With a density of
0.025-0.01 nozzles per square metre, and
6 h of daily operation, they involve a water
use of 0.6-18 l m
−2
day
−1
(Urban, 1997a).
This system is cheaper and has fewer
blockage problems although it may wet the
plants, leaving deposits over the leaves, and
dripping at the beginning and at the end of
each fogging episode.
8.7
Shading
The limitation of solar radiation, as a means
to avoid high temperatures in the green-
house, is that it involves a concomitant
decrease in photosynthesis, which in turn
involves a yield decrease.
The use of 'cooling' films, which limit
the input of IR radiation from the Sun
inside the greenhouse without affecting the
PAR range, could be a solution when their
efficiency in limiting the IR radiation is
improved and their use is economical (see
Chapter 4).
The shading devices can be outside or
inside the greenhouse. External shading
screens are preferable from the energy point
of view, as they avoid the heat input in the
greenhouse (Photo 8.6), but they must resist
atmospheric agents (wind, degradation,
loads such as dust or dirt, hail or snow car-
ried in the air).
The placement of wood sheets, reeds or
similar materials over the cover, which
could be rolled back and forth as required,
was used decades ago and this technique is
still popular in some countries like China
(see Fig. 7.5). However, due to their sensi-
tivity to the wind and difficulty of manage-
ment, in most countries nowadays they
have usually been replaced with shading
screens that have good mechanical resist-
ance and that are fixed using a large range of
fixing systems (Fig. 8.11).
The whitening of the cover with differ-
ent products that reflect the radiation is a
usual practice during high radiation periods.
The duration depends on the characteristics
Air/water systems
In these systems, there are two circuits: (i) a
low pressure circuit for the water (with an
operation pressure between 0.2 and 0.6 MPa)
and another one for compressed air (between
0.2 and 0.35 MPa).
The water and the air are canalized to the
interior of an atomizer which spreads the
flow into small droplets. The size of the drop-
lets and the flow of the nozzles is a result of
the pressure differences between the air and
water flows. The air pressure must necessar-
ily be, at least, equal to that of the water to
achieve water drops smaller than 10 mm (fog-
ging). If the air pressure is lower, the water
droplet size is greater than 50 mm (diameter),
with flows of up to 50 l h
−1
.
The best results are obtained with
pressures of 0.2-0.25 MPa for the water
and 0.3-0.35 MPa for the air, the consump-
tion being similar to those of the high pres-
sure systems (6-7 l h
−1
), with densities of
0.06-0.1 nozzles per square metre (Urban,
1997a).
This system has fewer blockage prob-
lems and it is easier to install but it is
more expensive to install than the high
pressure system because it needs a com-
pressor. It must be well regulated, drops
fall at the beginning and at the end of each
fogging episode and it uses quite a lot of
energy.
